The present invention relates generally to nuclear magnetic resonance (NMR) imaging apparatus and, more particularly, to high performance receiver coils for NMR Imaging of a human head.
Nuclear magnetic resonance (NMR) imaging is a known technique for obtaining cross sectional images through desired portions of a human body without exposing a patient to ionizing radiation. Briefly, the patient is placed in a static magnetic field which causes magnetic dipoles of atomic nuclei with spin (for example, hydrogen nuclei) to orient themselves with the magnetic lines of force. Like a spinning gyroscope, the spinning nuclei tend to precess with a certain angular frequency, known as the Larmor frequency. By means of a transmitter coil a radio frequency pulse is applied at a frequency which matches the natural precessional frequency. This causes the magnetic dipoles to quickly precess, while absorbing energy. When the excitation pulse ends, the nuclei briefly emit an RF signal known as the free induction signal or free induction decay. The magnetization vector of the nuclei eventually returns to its original position. This emitted RF energy can be detected by inductive coupling to a receiver coil and analyzed in view of the nature of the excitation pulse to build a set of data from which images can be constructed.
It will be understood and appreciated that the present invention is not directed to any of the various techniques for defining the cross sections for which images are constructed (e.g. phase encoding techniques) and the techniques for actually constructing the images. Rather, the present invention assumes the existence of these known techniques. In general, the present invention is concerned with efficiently receiving the emitted RF energy from a region of the body in bulk to provide a signal suitable for analysis.
It is possible for a single coil to serve the transmitter and receiver functions. However, for optimum performance it is desirable to provide separate transmitter and receiver coils due to different design considerations.
For example, the transmitter coil should be large, have good RF homogeneity, and relatively low Q to provide broadband excitation.
The receiver coil, however, should be as small as possible consistent with the region of interest, have a reasonable amount of RF homogeneity, have a high Q (narrow bandwidth), be minimally sensitive to dielectric loading, be comfortable for the patient, and be easy to use so as to provide the minimum connection and set-up time. An RF receiver coil should maximize flux coupling from the patient to the coil surface, and at the same time, in order to prevent damage to the associated preamplifier, be reasonably orthogonal to the transmitter coil so as to minimize coupling between the two coils, and should also function in a plane perpendicular to the main (static) magnetic field.
One important performance criterion of a tuned receiver coil is the signal-to-noise ratio (SNR) which can be obtained. In theory, SNR can be calculated by formula based on the frequency, the magnetic field due to a unit current in the coil, sample volume, loaded circuit Q, filling factor, and resistive loss due to coil impedance. The first three of these variables are easily determined, but the others are not.
In a receiver coil design the specific part of the anatomy is usually considered the starting point. Theoretical analysis is, however, limited by several factors. First, it is difficult to mathematically determine the ideal coil geometry for each organ of interest, giving rise to a large number of possible shapes. Even simple application of Faraday's law of induction may be inappropriate. For example, a multiple turn solenoid may be no better than a single turn solenoid of the same diameter when loaded with a biological sample. Electrical performance criteria such as coil Q may become irrelevant when comparing different coil geometries since Q is simply the ratio of inductive reactance to coil resistance, and inappropriate geometries may have good Qs.
The case of a human head may, at first impression, seem trivial, since it may be viewed as a nicely isolated sphere connected at one end (i.e. the neck), making it feasible to use both surface coil geometries and volume coil geometries.
Worthy of initial consideration is the question of why not use the most ideal coil geometry, the simple solenoid, on the head. The answer to this question depends on the main field orientation of the overall possible in the case of NMR imaging systems where the magnetic field is vertically oriented (assuming the patient is lying horizontally). In such cases, a solenoid receiver coil provides a maximum sensitivity and homogeneity. However, there are few systems where it is practical to apply such a coil.
In the case of the more commonly encountered horizontal bore systems, (most resistive magnet systems and all super conducting magnet whole body systems), the simple solenoid is not applicable because its proper orientation would require that turns of the coil pass through the neck. As a result, this otherwise ideal geometry has been passed by in favor of several other geometries, including the saddle coil, the double elliptical coil, the half saddle, the Alderman Grant resonator, and the slotted tube resonator.
The saddle coil has been the most widely accepted solution to the problem of coil orientation. However, it is clearly not as efficient as the solenoid, even though it does provide good homogeneity and reasonable sensitivity if constructed properly. There also exists a half saddle geometry which has the advantage of an open top which makes patient use extremely easy. The patient simply lies down on a cradle which has the half saddle coil within. Unfortunately, this design also suffers loss in sensitivity when compared to the conventional fully saddle geometry.
In summary, during the course of NMR imaging, energy is inductively coupled from the patient to an RF receiver coil, where the emf induced in the coil and therefore the SNR available for image construction depends in part on flux coupling between the patient and the coil. Current low field NMR Imaging Systems are limited to thick slice (1 cm) medium resolution images of the brain due partly to RF coil inefficiency; SNR requirements for thin section 0.4 to 0.6 cm images are 40% to 60% higher. Numerous RF coil designs have bee suggested in the literature, including solenoids, saddle coils, crossed elliptical, toroidal, and slotted tube resonators. Solenoids are perhaps the oldest, but they are inappropriate in a magnet with a horizontal bore since they must be oriented in a vertical axis. While the crossed elliptical coil and the toroid overcome the orientation problem, they are much less efficient than a solenoid. While a slotted tube resonator is nearly as efficient as a solenoid, it has such low inductance that it is difficult to make it perform properly at the frequencies involved in a 0.15 T NMR imaging system (wherein the frequency of interest is 6.25 MHz). Thus, previously proposed coils are inadequate.